Depending upon the application, generally it is important bring a fluid into intimate contact with the surface of the filter medium during filtering. This is particularly true when a membrane separator are employed in the filtration process. A membrane separator can function in a manner that filters out particulates, separates liquids from gases, and in some cases separate immiscible liquids from one another.
A common technique employed when sampling a fluid stream under pressure, particularly in analytical applications, is to have the fluid enter a housing, and have a portion of the fluid entering the housing bypass the filter medium. This procedure is followed in order to decrease transport time and purge incoming sample lines.
This procedure also serves, in many cases, to remove some of the materials that were separated from the incoming fluid by the filter medium. This is particularly so in membrane separators, wherein liquids are rejected by the membrane and are swept out with the bypass stream.
Utilizing this technique, it is also possible to remove particulates continuously as they accumulate on the surface of the filter medium or membrane. The filtrate leaves the housing through an outlet port. In the case of analyzers, the sample may be further conditioned after passing through the filter medium or membrane before being sent to an analyzer.
Past filter designs often have unnecessary internal volume, which provides problems in use. It would be desirable, particularly in analytical applications, to keep the internal volume of a filter housing at a minimum and configure the internal volume free of unpurged areas or “dead volume”. Another desirable characteristic of a filter housing would be to provide easy access to the filter medium.
For safety and environmental reasons, it would be highly desirable that the filter medium be accessible without disconnecting any of the fluid lines attached to the filter housing.
Other prior designs have offered poor performance due to inconsistent pressure differentials applied across the filter medium. In cases where phase separating membrane are employed, it is therefore important to limit the pressure differential across the membrane. If the pressure difference between the upstream and downstream sides of the membrane exceed certain values (which may vary with individual application), then some liquid which would be otherwise rejected will pass through the membrane, and the phase separation membrane will not perform as designed.
Accordingly, it is desirable to provide a sample system in such a manner that will minimize the risk of exceeding recommended membrane differential pressures.
It can be a costly and difficult task to design sample systems in the manner required to prevent excessive membrane differential pressures from occurring under all conditions. Further, this process is not well understood by all who design sample systems.
For example, a system that maintains a suitable membrane differential pressure under normal circumstances may not prevent excessive membrane differential pressures from occurring during upset conditions. This is a frequent occurrence which allows some of the undesired liquid to flow through the membrane. The net result could be that an analyzer is damaged and/or analysis compromised.
One type of current art housing is comprised of two plates arranged to hold the sheet of filter media or membrane between the interior plate surfaces in a parallel configuration. The plates are held together with bolts or screws. Refer to Mayeaux U.S. Pat. No. 5,476,586. The fluid entry, exit, and bypass ports are distributed on both plates.
This type of construction requires detachment of one or more fluid lines from the housing in order to access the membrane, which by necessity interrupts the flow of fluid through the housing. In many cases this presents safety and environmental hazards.
Another type of current art housing is comprised of a hollow body, filter media holder and cover. The entry, exit, and bypass ports are all located in the body segment of the housing. The filter media or membrane is retained in the holder segment which is inserted into the body segment. The cover is threadingly engaged to the body segment and retains the media holder inside of the body segment.
This type of three-piece construction is more expensive to manufacture and requires the removal of two housing parts to access the filter media or membrane.